1. Field of the Invention
This invention relates to an illuminating optical system for a microscope, and more particularly to a condenser optical system in a microscope for illuminating a sample surface.
2. Description of the Prior Art
Generally, an illuminating optical system for a microscope is indispensable for various microscopic examinations using a phase microscope, an interference microscope or the like. Above all, the condenser lens not only performs the function of concentrating the illuminating light upon a sample surface but also bears an important role of compensation for various physico-optical actions. For effective illumination where the objective lens is of high magnification, an optical system in which the magnification of the condenser lens itself is high, namely, the focal length is short, is advantageous.
However, from the view point of the light source compensation, particularly in the polarization type differential interference microscope, the shorter the focal length of the condenser lens, the more difficult providing compensation for the light source becomes. That is, for the compensation for the light source of the polarization type differential interference microscope, a member identical to a so-called Wollaston prism disposed rearwardly of the objective lens is located at the position of the pupil of the condenser lens. However, this Wollaston prism is for splitting polarized light and structurally it is not essentially coaxially symmetrical with respect to the optic axis. Accordingly, the manner in which the prism acts upon the received light differs depending on the direction of the incident light. Moreover, the shorter the focal length of the condenser lens, the greater the angle of incidence of the light ray upon a Wollaston prism placed at the front focus position of the condenser lens. Therefore, the anisotropy of the action of the prism on the light becomes more significant so that the light source compensation becomes incomplete.
Thus, the use of the conventional condenser lens capable of effecting illumination of high magnification with a polarization type differential interference microscope results in incomplete compensation for the light source, and this has made it impossible to observe the more minute structure of a sample in a uniform view field.